This invention generally relates to any controllable motor and to a current regulation circuit for controlling circulating currents in the motor and, particularly, to a current regulation circuit for detecting and controlling circulating currents in a controllable, variable speed motor with a bi-directional current sensing circuit.
A motor system according to the present invention typically includes a brushless DC motor having an inverter bridge for driving the motor. The inverter bridge has a number of power switching devices with a flyback diode coupled to each of the devices and is used to connect the motor's windings to a power supply through a power supply link. The power supply link has positive and negative rails connecting the motor to the power supply. Generally, two windings of a three-phase motor are energized at a time and the power is pulse width modulated for control purposes. Such a motor system provides pulse width modulation (PWM) of the power to the windings by turning on and off one of the power switching devices. Under the normal PWM control scheme, the motor system turns off one device at a time which causes a flyback current to circulate in the inverter bridge. Such circulating currents result from energy stored in the phase winding inductances and flow through one power device and one diode in the inverter bridge. In normal operation, circulating currents decay rapidly because the motor back electromotive force (EMF) opposes their flow. In abnormal operation, however, circulating currents are out of position and the back EMF voltage causes them to increase rather than to decay which can result in damage to the current-carrying power devices as well as to the motor's permanent magnets. A momentary reversal of rotation at start-up or being out of position due to a failure to properly sense rotor position are examples of abnormal operation causing increasing circulating currents.
Generally, current sensors are required in all phases of the motor to determine what currents are in the motor. Further, systems employing only a single current sensing element are unable to detect circulating currents because they cannot be observed in a shunt resistor (or other current sensor) in the power supply link.
Brushless DC motors are disclosed in U.S. Pat. No. 4,757,241 and application Ser. No. 08/108,328, filed Aug. 18, 1993, both of which are commonly assigned with the present application and the entire disclosures of which are incorporated herein by reference in their entirety.